Tilted grating sensor

ABSTRACT

The present invention relates to a sensor using a tilted fiber grating to detect physical manifestations occurring in a medium. Such physical manifestations induce measurable changes in the optical property of the tilted fiber grating. The sensor comprises a sensing surface which is to be exposed to the medium, an optical pathway and a tilted grating in the optical pathway. The grating is responsive to electromagnetic radiation propagating in the optical pathway to generate a response conveying information on the physical manifestation.

FIELD OF THE INVENTION

The invention relates to a sensor using a tilted fiber grating to detectexternal events that induce measurable changes in an optical property ofthe tilted fiber grating. The sensor can be used to sense a wide rangeof biological processes, biological elements, chemical elements ormanifestations of physical phenomena such as temperature, strain orindex of refraction.

BACKGROUND OF THE INVENTION

Fiber Bragg grating (FBG) sensors have a wide range of applications suchas pressure-strain sensors, temperature sensors, micro-bending sensorsand external refractive index sensors. As these optical sensors areinherently immune from electromagnetic interference and chemicallyinert, they are very attractive in bio-chemical applications andhazardous surroundings.

The sensing mechanism most often used in FBGs arises from the fact thatthe reflection wavelength for the forward propagating core mode varieslinearly with temperature and strain. Since the wavelength can bemeasured with an accuracy of 10 pm relatively easily near 1550 nm, thisrepresents a relative resolution of about 6 ppm. A variant of the sameconcept uses so-called Long Period Gratings (LPG) where coupling occursbetween the forward propagating core mode and forward propagatingcladding modes. In this case, the sensitivity of the resonancewavelength to perturbations can be greatly enhanced for some of thecladding modes. Furthermore, since LPGs involve cladding modes there hasbeen great interest in using these for refractive index sensing byimmersing the fibers in the medium to be measured. Special absorbingcoatings can also be used to detect chemicals or liquids through therefractive index changes (or volume changes) induced in the coatings.However, the spectral response of LPG resonances is rather broad (widthgreater than 10 nm) making high accuracy measurements of smallwavelength changes more difficult than with FBGs. Ideally, suchrefractive index sensors should be able to distinguish different kindsof perturbations, and insensitivity to temperature is often particularlydesirable. A problem with both FBG and LPG sensors is that they areintrinsically quite sensitive to temperature, with resonance wavelengthsdrifting by about 10 pmfC, unless special bulky packaging is used toathermalize the device. In order to circumvent this problem inrefractive index sensors, devices proposed so far have involvedcombination of gratings in one sensor such as two different types offiber Bragg gratings, two fiber Bragg gratings with different claddingdiameters and a long period grating (LPG) with a Bragg gratings. In suchcases, the differential sensitivity of the two gratings to temperatureand the desired measurand is used to discriminate between the twoperturbations.

Against this background it can be clearly seen that the current sensortechnology has drawbacks. It is therefore the aim of the presentinvention to alleviate those drawbacks.

SUMMARY OF THE INVENTION

As embodied and broadly described herein the invention provides a sensorfor sensing at least one physical manifestation occurring in a medium.The sensor comprises a sensing surface for exposure to the medium, anoptical pathway and a tilted grating in the optical pathway. The gratingis responsive to electromagnetic radiation propagating in the opticalpathway to generate a response conveying information on the at least onephysical manifestation.

As embodied and broadly described herein the invention also provides asensor for sensing a physical manifestation occurring externally of thesensor. The sensor comprises a sensing surface for exposure to thephysical manifestation, an optical pathway and a tilted grating in theoptical pathway. The tilted grating is responsive to electromagneticradiation propagating in the optical pathway to induce SPR adjacent thesensing surface.

As embodied and broadly described herein the invention also provides amethod for detecting the presence of bacteria in a medium. The methodcomprises providing a sensor having a sensing surface, the sensor alsohaving an optical pathway containing a tilted grating, placing thesensing surface in contact with the medium and determining from aresponse of the sensor if the bacteria is present in the medium.

As embodied and broadly described herein the invention also provides amethod for detecting the presence of virus in a medium. The methodcomprises providing a sensor having a sensing surface, the sensor havingan optical pathway containing a tilted grating, placing the sensingsurface in contact with the medium and determining from a response ofthe sensor if the virus is present in the medium.

As embodied and broadly described herein the invention also provides amethod for measuring the concentration of sugar in a medium. The methodcomprises providing a sensor having a sensing surface, the sensor havingan optical pathway containing a tilted grating, placing the sensingsurface in contact with the medium and determining from a response ofthe sensor the concentration of sugar in the medium.

As embodied and broadly described herein the invention also provides amethod for measuring the concentration of alcohol in a medium. Themethod comprises providing a sensor having a sensing surface, the sensorhaving an optical pathway containing a tilted grating, placing thesensing surface in contact with the medium and determining from aresponse of the sensor the concentration of alcohol in the medium.

As embodied and broadly described herein the invention also provides amethod for detecting the presence of a chemical or biological element ina medium. The method comprises providing a sensor having a sensingsurface, the sensor having an optical pathway containing a tiltedgrating, placing the sensing surface in contact with the medium anddetermining from a response of the sensor if the chemical or biologicalelement is present in the medium.

As embodied and broadly described herein the invention also provides amethod for measuring a degree of curing of a curable material. Themethod comprises the steps of providing a sensor having a sensingsurface, the sensor having an optical pathway containing a tiltedgrating, placing the sensing surface in contact with the curablematerial determining from a response of the sensor the degree of curingof the curable material.

As embodied and broadly described herein the invention also provides anelongation strain sensor. The elongation strain sensor comprises anoptical pathway and a tilted grating in the optical pathway to generatea response conveying information on elongation strain acting on thesensor.

As embodied and broadly described herein the invention also provides amethod for measuring elongation strain. The method comprises receiving aresponse from a sensor containing a tilted grating subjected toelongation strain, the response conveying information on reaction of thetilted grating to elongation strain and on reaction of the tiltedgrating to temperature, processing the response of the tilted grating todistinguish the reaction of the tilted grating to elongation strain fromthe reaction of the tilted grating to temperature.

As embodied and broadly described herein the invention also provides anapparatus for measuring elongation strain. The apparatus comprises anelongation strain sensor having an optical pathway, a tilted grating inthe optical pathway to generate a response conveying information onelongation strain and temperature acting on said sensor and a signalprocessing unit to process the response of the tilted grating anddistinguish in the response to reaction of the tilted grating toelongation strain from the reaction of the tilted grating totemperature.

As embodied and broadly described herein the invention also provides abending strain sensor. The bending strain sensor comprises an opticalpathway and a tilted grating in the optical pathway to generate aresponse conveying information on bending strain acting on the sensor.

As embodied and broadly described herein the invention also provides amethod for measuring bending strain. The method comprises receiving aresponse from a sensor containing a tilted grating subjected to bendingstrain, the response conveying information on: reaction of the tiltedgrating to bending strain; reaction of the tilted grating totemperature. The method also comprises processing the response of thetilted grating to distinguish the reaction of the tilted grating tobending strain from the reaction of the tilted grating to temperature.

As embodied and broadly described herein the invention also provides anapparatus for measuring bending strain. The apparatus comprises abending strain sensor, having an optical pathway, a tilted grating insaid optical pathway to generate a response conveying information onbending strain and temperature acting on said tilted grating, a signalprocessing unit to process the response of the tilted grating anddistinguish in the response to reaction of said tilted grating tobending strain from the reaction of the tilted grating to temperature.

As embodied and broadly described herein the invention also provides apressure sensor. The pressure sensor comprises an optical pathway, atilted grating in the optical pathway to generate a response conveyinginformation on bending strain acting on said sensor, a flexible member,the optical pathway being mounted to the flexible member. The flexiblemember flexes in response to pressurized fluid and induces in the tiltedgrating bending strain.

As embodied and broadly described herein the invention also provides asensor, for sensing at least one physical manifestation occurring in amedium. The sensor comprises an optical pathway an interface coupledwith the optical pathway, the interface being responsive to the physicalmanifestation to induce strain on said optical pathway; a tilted gratingin the optical pathway, the tilted grating being responsiveelectromagnetic radiation propagating in the optical pathway to generatea response conveying information on the strain induced on the opticalpathway.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of examples of implementation of the presentinvention is provided hereinbelow with reference to the followingdrawings, in which:

FIG. 1 a is a high level schematic of a TFBG sensor;

FIG. 1 b is a graph showing the simulated transmission spectrum of aTFBG;

FIG. 2 a is a graph showing the differential wavelength shift forselected TFBG resonances as a function of the refractive index of thesurrounding medium for a 125 μm cladding diameter;

FIG. 2 b is a graph showing the differential wavelength shift forselected TFBG resonances as a function of the refractive index of thesurrounding medium for an 80 μm cladding diameter;

FIG. 3 is a graph showing the experimental TFBG transmission spectrumfor three values of SRI;

FIG. 4 is a graph showing the temperature sensitivity of several TFBGresonances and differential shift from the core mode resonance;

FIG. 5 is a graph showing the effect of bending of a TFBG on itstransmission spectrum;

FIG. 6 is a scheme for interrogating an individual cladding moderesonance using a photo-detector collecting all the reflected light. Theinsets in the figure indicate the transmission spectrum of a typicalTFBG (near a cladding mode resonance) and the reflection spectrum of theinterrogating Fiber Bragg Grating (FBG). The position of the FBGreflection peak determines which cladding mode is interrogated.

FIG. 7 is a graph showing the displacement of the reflected power from aTFBG-FBG pair when the FBG is tension-tuned across a TFBG cladding moderesonance.

FIG. 8 is a graph showing the transmission spectrum of a TFBG;

FIG. 9 is a cross-sectional view of optical fiber containing a TFBG thatproduces surface plasmon resonances;

FIG. 10 shows a dielectric/conductor interface on an optical fiber toillustrate the Kretchman configuration for exciting SPR.

FIG. 11 is a graph illustrating the transmission spectrum of the samegrating as the one of FIG. 8, but with a 20 mm gold layer in air.

FIG. 12 is a graph showing the transmission spectrum of gold-plated TFBGin a sugar solution. The bracket identifies the peak position of theanomalous resonance.

FIG. 13 is a graph showing the SPR peak wavelength (λ_(p)) andcorresponding cladding mode effective index on the refractive index ofthe external medium at 589 nm (n_(D))

FIG. 14 is a block diagram of a measuring apparatus using a tiltedgrating, according to an example of implementation of the invention.

FIG. 15 is a block diagram of the signal processing device of themeasurement apparatus shown in FIG. 14.

FIG. 16 is a scheme of a diaphragm 2000 that is made of flexiblematerial and exposed to pressure on its side 2002. A sensor 2004 ismounted to the diaphragm. The sensor is placed on the convex side of thediaphragm 2000.

FIG. 17 is a schematic representation of self-assembled monolayersmolecules on the sensing surface. The self-assembled monolayersmolecules comprises a recognition analyte which associates with ananalyte of interest.

FIG. 18 is a block diagram of a measuring apparatus using a tiltedgrating, according to an example of implementation of the invention. Theapparatus comprises a reflector as well as an optical coupler.

In the drawings, embodiments of the invention are illustrated by way ofexample. It is to be expressly understood that the description anddrawings are only for purposes of illustration and as an aid tounderstanding, and are not intended to be a definition of the limits ofthe invention.

DETAILED DESCRIPTION

A non-limiting example of implementation of this invention uses anoptical structure in which light is guided by a core medium able tosupport one or few guided modes, surrounded by a finite-sized claddingmedium whereas the cladding itself acts as a multimode waveguide.Specific examples of this structure include optical fibers and planarlight circuit (PLC). In a weakly tilted fiber Bragg grating (TFBG)sensor both a core mode resonance and several cladding mode resonancesappear simultaneously, as shown in FIG. 1. Weakly tilted gratings aredefined as having a tilt angle greater than zero but less than 45degrees relative to the propagation axis, so that there is a non-zerocore mode reflection induced by the grating. In a specific example, thetilt angle is in the range from greater than zero and about 20 degrees.In a more specific example the tilt angle is in the range from about 2degrees to 12 degrees. This has several advantages. The cladding moderesonances are sensitive to the external environment (refractive index,deposited layer thicknesses, etc.) and to physical changes in the wholefiber cross-section (shear strains arising from bending for instance),while the core mode (Bragg) resonance is only sensitive to axial strainand temperature. The temperature dependence of cladding modes is similarto that of the core modes, so that the effect of temperature can beremoved from the cladding mode resonance by monitoring the wavelengthdifference between the core mode resonance and selected cladding moderesonances. Using this technique, sensors can be made for sensingphysical manifestations such as elongation strain, bending, or measuringthe Surrounding Refractive Index (SRI), that aretemperature-independent.

Another feature which may constitute an advantage of the TFBG over theLPG to couple to cladding modes is that the resonances are as narrow asthose of a FBG, i.e. of the order of a few hundred pm, instead ofseveral tens of nm for LPGs. Therefore, wavelength interrogation forTFBG occurs over narrow wavelength ranges and makes it possible to usecommonly available light sources, detectors, couplers and multiplexers.In particular, a typical entire TFBG spectrum fits easily within thestandard telecommunication bands (1530-1560 nm). Also, by using theresonance positions and/or strengths individually, allows extractingmultiple sensing parameters from a single sensor. More specifically, amulti-functional sensor can be built by monitoring several cladding moderesonances (or groups of resonances) which have different sensitivitiesto different physical manifestations. For instance, bending affectsmainly low order cladding modes, while SRI variations affect higherorder cladding modes preferentially, and neither perturbation affectsthe core mode resonance.

As is well known, the Bragg reflection and cladding mode resonancewavelengths λB and λiclad of TFBG are determined by a phase-matchingcondition and can be expressed as follows:λ_(B)=2n _(eff)Λ/cos θ  (1)λ^(i) _(clad)=(n ^(i) _(eff) +n ^(i) _(clad))Λ/cos θ  (2)where neff, nieff and niclad are the effective indices of the core modeat λB and the core mode and the ith cladding mode at λicladrespectively, and Λ and θ are the period and the internal tilt angle ofthe TFBG. The tilt angle is defined as the angle formed by the TFBG andthe imaginary axis of the optical pathway containing the TFBG alongwhich the optical signal interrogating the TFBG propagates. When theoptical pathway is defined by an optical fiber, the axis of the opticalfiber will usually constitute the axis of optical signal propagation.For TFBG structures, if only the Bragg and cladding mode wavelengthshifts (ΔλB, Δλiclad) caused by external refractive index changes(Δnext) and temperature changes (ΔT) are taken into account, thewavelength shifts ΔλB and Δλiclad can be written from equations (1) and(2) as follows:

In standard optical fibers, the effective index of core modes (n_(eff),n^(i) _(eff)) are insensitive to the external refractive index changes,so the equations (3) and (4) may be simplified as following:

Equations (5) and (6) show that the cladding resonances will change withthe external refractive index, but not the Bragg wavelength. It can alsobe seen that the different cladding modes may have differentsensitivities to SRI changes. The differential wavelength shift, whichcan be used as a sensing quantity, is given by Equation (7). The secondterm on the right-hand-side of Equation (7) represents the temperaturedependence of the relative wavelength shift. Using the thermal expansioncoefficient of silica (0.55×10⁻⁶/° C.) and an extreme worst caseestimate of 1×10⁻⁶/° C. for the difference in the temperature dependenceof the refractive indices of the core and cladding glasses (which changeindividually by about 10×10⁻⁶/° C.)), it can be shown that thetemperature dependence of the differential resonance is less than 0.54pm/° C. for the differential wavelength shift.

Accordingly, the value of Δn_(ext) determined from Equation (7) is thevalue for the SRI at the sensing temperature (and the temperature can bedetermined independently by the core mode resonance value from Equation(5)).

A number of simulations of TFBG constructions will now be discussed toillustrate the main features of the TFBG sensor. Those simulations havebeen made using a commercial optical fiber grating software simulator,such as OptiGrating version 4.2 from Optiwave Corp.,(http://www.optiwave.com).

First, a TFBG with grating period of 534 nm and internal tilt angle of4.5° was simulated in a CORNING SMF-28 standard optical fiber as afunction of the refractive index of the external medium (SRI). Thestructure of such TFBG grating is shown at FIG. 1 a. Broadly speaking,the TFBG is integrated into an optical pathway 10 which is in the formof an optical fiber. The optical pathway has a core 12 surrounded by acladding 14. The TFBG grating 16 is written into the core 12. The axisof the TFBG 16, which is perpendicular to the bars forming the grating16 is at an angle of 4.5° with relation of the axis 18 of the opticalpathway 12, along which an optical signal interrogating the TFBG 16propagates. When the TFBG 16 is interrogated by an optical signal theTFBG 16 produces a response that has two components. One of thecomponents is the core mode resonance 20 which is reflected back towardthe source of the optical signal. The other is the cladding moderesonance 22 which includes a series of individual emissions atdifferent wavelengths that propagate in the cladding 14 toward the outersurface of the optical pathway.

FIG. 2( a) shows the wavelength shift of 5 different cladding moderesonances (relative to the core mode resonance, which remains fixed)when the SRI changes from 1.33 to 1.44. The highest order cladding modesbegin to shift sooner as the SRI increases because their mode fieldintensity extends further into the external medium as they are closer totheir cut-off. High order modes sequentially disappear (from the shortwavelength side, as shown in FIG. 1 b) when the SRI reaches values forwhich they are cut-off.

Note that the final slope before cut-off for the various resonancesappears to converge for all the modes to a common value near 180 pm/%SRI. Therefore, monitoring several resonances simultaneously allows highsensitivity measurements of SRI over a larger range of values.

Higher sensitivity also can be achieved by reducing the cladding layerthickness (by etching in diluted hydrofluoric acid for instance), or byusing fibers that are fabricated with smaller diameter claddings: thisresults in fewer cladding modes that are more widely separated inwavelength. If the cladding layer diameter is reduced from 125 μm to 80μm, the SRI sensitivity becomes 350 pm/% SRI, as shown in FIG. 2( b). Byfurther reducing the cladding layer diameter, much higher wavelengthshifts can be expected. Note that a potential problem may arise if thecladding layer diameter is reduced to less than 30 μm; at or near thisvalue the Bragg wavelength becomes sensitive to the SRI changes and thein-fiber temperature reference may be lost. Another option is to usefunctional thin films to “pull” certain modes out of the fiber claddingand make them sensitive to specific environments.

The results of experimental work conducted for SRI sensing andtemperature insensitivity are shown in FIGS. 3 and 4 respectively. TheTFBG is written in a CORNING SMF 28 fiber using ArF excimer laser lightat 193 nm and a phase mask to generate the grating pattern. This is notto be considered a restriction since any method of fabrication for FBGsis applicable to the sensors described here. The tilt angle was adjustedexperimentally until the core and main cladding mode resonancetransmission dips achieved comparable attenuation levels. The exactvalue of the tilt angle is not a critical parameter as it onlyinfluences the relative strengths of the core and cladding moderesonances for a given sensor (and hence only impacts the details of thesensor interrogation scheme chosen in a particular case). The SRIsensitivity was obtained by measuring the transmission spectra of theTFBG immersed in calibrated refractive index fluids (from CargilleLaboratories). FIG. 3 shows the changes in transmission of the highestorder cladding modes with SRI of 1.40, 1.42 and 1.44, illustrating thatresonance shifts increase with mode order and that large changes occurwhen modes approach cut-off.

The measured wavelength shifts of modes approaching cut-off are 0.275 nmand 0.218 nm when the SRI changes from 1.40 to 1.42 and 1.42 to 1.44respectively. The latter shift corresponds to a sensitivity of 300 pm/%SRI, actually higher than the theoretical values indicated previously.If the power level within a narrow band at a fixed wavelength ismonitored instead of the wavelength itself, the above mentionedwavelength sensitivity translates into a change in transmitted power of20% for a change in SRI of 0.7% since resonances are at most 200 pm widefor 1 cm-long gratings, and several resonances have an amplitude ofabout 1 dB. By monitoring simultaneously the power level near the coremode resonance and assuming that 0.1% level difference between the twomeasurements can be detected, the minimum detectable SRI is of the orderof 4×10⁻⁵. It is important to note that the first resonance on the shortwavelength side of the core mode Bragg reflection is a ghost mode whichdoes not appear in simulations unless a UV induced break in symmetry orrelatively large core index increase is included in the calculation.This ghost mode may be useful in detection of fiber bends and shearstresses.

As mentioned above, it is desirable that the wavelength shifts measuredin SRI sensing do not depend on the temperature of the sensor. Theexperiments show that while individual resonances of core and selectedcladding modes each drift by ˜10 pm/° C. (for the first resonance (LP₀₁:Bragg wavelength resonance), the second resonance (ghost mode resonance)and the seventeenth resonance), as shown in FIG. 4, the relativewavelength shift between the cladding modes and the Bragg wavelength isless than 0.4 pm/° C. This represents a ˜27 times reduction insensitivity from the resonance wavelengths themselves.

This confirms the usefulness of the self-referencing feature for makingthis SRI sensor temperature-independent.

Finally, the experimental results show that the TFBG spectrum changes asa function of bending in FIG. 5 (using a stronger grating written inhydrogen-loaded fiber). As expected since the fiber core lies on aneutral strain axis for pure bending, the core mode resonance and highorder cladding modes remain unchanged but the first few low ordercladding mode resonances change by several dB for the bending radiitested. Therefore, a single sensor can be used to detect three differentphysical manifestations simply by interrogating it over three differentwavelength ranges: 1) near the Bragg resonance for temperature; 2) nearlow order cladding modes for bending; 3) at the short wavelength end ofthe transmission spectrum for SRI.

A particularly simple interrogation scheme for the TFBG could beconstructed by spectrally slicing the transmitted light using an arrayedwaveguide grating (AWG) butt-coupled to a detector array. The spacingand widths of the resonances are quite compatible with standard 40 or 80channels AWGs that are widely available from telecommunicationscomponent vendors. Instead of tracking wavelength shifts, the detectorsmonitor power level changes at fixed wavelength positions. One channelof the AWG interrogator can be used in closed loop to thermally tune theAWG (forcing its wavelength comb to follow the core mode resonance)while the output of another channel would reveal the power level changesassociated with the relative wavelength shift of a cladding moderesonance. Level discretization and digital processing can then be usedto extract meaningful data from the TFBG response. For more informationon this interrogation scheme, the reader is invited to refer to thearticle of G. Z. Xiao, P. Zhao, F. G. Sun, Z. G. Lu, Z. Zhang, and C. P.Grover, “Interrogating fiber Bragg grating sensors by thermally scanninga demultiplexer based on arrayed waveguide gratings,” Opt. Lett., vol.29, pp. 2222-2224, 2004. The content of this article is incorporatedherein by reference.

Another scheme, requiring only a power detector, may be used tointerrogate a selected cladding mode resonance independently oftemperature or strain. This involves a pair of gratings as shown in FIG.6. The TFBG is placed at the sensing point and is followed by a highreflectivity regular FBG of the same length (hence having the samespectral bandwidth) but with a Bragg wavelength AB centered near anyoneof the cladding mode resonances (referred to as λC). Incident broadbandlight goes through the TFBG and gets attenuated at λC, continues to theFBG, gets reflected and returns towards after having passed a secondtime through the attenuating TFBG. This reflected light is recuperatedfrom the fiber using either a circulator or a 3 dB coupler. The spectralbandwidth of the input light needs only to be large enough to cover themaximum wavelength excursion of the sensor in operation. The totalamount of light reaching the detector consists of the residual Braggreflection from the TFBG (which can be minimized for certain tiltangles) and the light reflected at λB. When λC=λB, very little lightreaches the detector but as soon as the cladding mode resonance shiftsor changes its strength the power level at the detector changes. As anexample, such a pair of gratings were fabricated with λC>λB initiallyand then the FBG was stretched (thereby increasing λB) through thecladding mode resonance. Light from a pumped erbium-doped fiberbroadband source was launched into the fiber containing the two gratingsthrough a 3 dB coupler. The reflected power detected near the input ofthe fiber as a function of strain on the FBG is shown in FIG. 7. Thegraph shows that the power changes by more than 10 dB when λB becomesequal to λC. This represents a simple scheme to detect changes in Braggwavelength or coupling strength without the need for an optical spectrumanalyser. As shown in FIG. 4, if the two gratings illustrated in FIG. 6are at the same temperature then λC and λB will move together and nopower level change will be observed in the reflected signal. In mostpractical applications it would be the FBG that would provide thereference and the power level fluctuations would reflect changes in theTFBG cladding modes due to external perturbations. However the scheme issymmetrical with respect to the two gratings and the experimentalresults discussed here show that the sensitivity can be quitesignificant (0.1 dB/μStrain).

In a possible variant, the TFBG is designed to produce plasmonresonances to perform measurements on physical manifestations. A typicalsetup is shown in FIG. 9. The TFBG 900 is fabricated in an optical fiber902. The cladding 904 of the optical fiber 902 is coated with a suitablemetal layer 906, such as gold to produce a dielectric/conductorinterface 910. Such dielectric/conductor interface 910 gives rise tosurface plasmon resonances. Specifically, in order for plasmonresonances to occur, the optical field within the fiber 802 has to havenon zero amplitude at the dielectric/conductor interface 910 and aspecific value of axial propagation constant (or, if one considers theray optics approach to wave guiding, of angle of incidence within thetotal internal reflection regime).

There are many potential applications for such structures, especially inthe optical sensing of chemicals, among others. The TFBG 900 is used tocouple the core mode light to a multitude of cladding modes, dependingon the light wavelength, as shown in FIG. 8. The cladding modes havenon-zero evanescent fields extending outside the cladding diameter andhence into the metal layer 906. When the axial component of thepropagation constant of the cladding mode equals that of an SPR wave,coupling to that SPR wave can occur. A single TFBG is sufficient togenerate a wavelength dependent set of cladding modes that are“interrogating” the metal layer 906 at various angles of incidence. Thisis most easily seen from the phenomenological representation shown inFIG. 9. FIG. 9 illustrates the ray optic analogy of the coupling from aguided core mode to several cladding modes through a TFBG. Each of themodes can be individually “addressed” simply by changing the wavelengthof the guided light, and each mode strikes the cladding boundary at adifferent angle of incidence. On the other hand, FIG. 10 shows thetraditional attenuated total reflection method (usually referred to asthe Kretschmann configuration) to excite and detect SPR waves bychanging the angle of incidence of the light beam incident on the metalfilm.

If there are non-radiative SPR waves that can be guided by the metalfilm surrounded on one side by silica glass and on the other side by asuitable medium, and if these SPR waves have an effective index (alongthe axis of the fiber) that is phase matched to one of the claddingmodes effective indices, then coupling can occur between this claddingmode and the SPR wave. When this occurs, the cladding modes involvedwill experience more loss than their neighbours. The effective index ofthe i^(th) cladding mode (n^(i) _(clad)), can be calculated from theresonance position λ^(i) _(clad) by the following expression:λ^(i) _(clad)=(n ^(i) _(eff) +n ^(i) _(clad))Λ/(cos θ)  (1)where n^(i) _(eff) is the effective index of the core mode at λ^(i)_(clad), and Λ and θ are the period and the internal tilt angle of theTFBG. The wavelengths of the cladding mode resonances that are perturbedas a result of a coupling to a SPR wave in a metal-coated TFBG, such asthe TFBG 900, the provide a direct measure of the effective index of theSPW through Equation (1).

In the course of experimental work fiber gratings were fabricated usingthe standard process of KrF excimer laser irradiation of hydrogen-loadedCORNING SMF28 fiber through a phase mask. The required tilt was achievedby rotating the mask-fiber assembly around an axis perpendicular to thefiber axis and to the plane of incidence of the laser light. Thetransmission spectrum of the grating used for the experiments reportedis shown in FIG. 8. The longest wavelength resonance corresponds to thereflection of the core mode light onto itself (Bragg wavelength), whileall the shorter wavelength resonances correspond to the excitation ofbackward propagating cladding modes. These modes are not reflected backto the source because they are rapidly attenuated by the fiber jacket assoon as they leave the grating region (where the jacket has been removedprior to fabricating the grating). For resonances between 1520 and 1560nm, phase mask periods of the order of 1 μm are used. After fabrication,the gratings were heat-stabilized by subjecting them to a rapidannealing at −300° C. and the remaining hydrogen removed by 12 hours ofheating at 120° C. prior to gold deposition. In these preliminaryexperiments, we used a small-scale sputtering chamber (PolaronInstruments model E5100) with the fiber positioned a few cm from thegold target. For flat samples in the same geometry, a gold layerthickness of 20 nm requires 1 minute of deposition at a pressure of 0.1Torr, a potential difference of 2.5 kV, and 18-20 mA of sputter current.In order to coat the fiber as uniformly as possible, two coating runswere made with the fiber holder rotated by 180 degrees between thecoatings. Under these conditions, the film uniformity around the fibercircumference is unlikely to be optimal. The film thickness on the fiberthat is indicated in this specification is the value expected for thetwo sides of the fiber that directly facing the sputtering target duringthe two coating runs. While thicknesses ranging from 10 to 50 nm weretested, the following description will focus on results obtained with a20 nm-thick nominal gold layer.

After the gold deposition, the fiber transmission spectrum is visiblymodified, but without measurable features of interest, as shown in FIG.11, indicating that the very thin gold layer has had an effect. However,no narrowband SPR resonances can be seen in the wavelength spectrumsampled by the cladding modes. When the gold-coated grating is immersedin liquids with various refractive indices (sugar solutions), anomalousresonances appear for certain very specific sugar concentrations, asdetermined from Abbe refractometer measurements of the refractive indexof the solutions at 589 nm (nD) (FIG. 12). The accuracy of the Abberefractometer that was used is of ±0.0002. These resonances are not thesame as those obtained for uncoated tilted fiber gratings, since theyhave a finite bandwidth within the cladding mode envelope and theirmaximum attenuation shifts rapidly with the external index. The peakposition of the anomalous resonance (λ_(p)) is obtained by fitting theenvelope of the cladding mode resonances. FIG. 13 shows how λ_(p)changes as the refractive index of the outer medium is increased bysmall amounts. The spatial width of the envelope of the anomalousresonances is about 5 nm.

By using equation (1) to find the effective indices of the claddingmodes within a resonance and the refractive index of silica near 1550 nm(n=1.444), it is possible to calculate the angular spread of theequivalent angles of incidences (since the effective index is equal tothe projection on the fiber axis of the refractive index in silica). Forthe data of FIG. 12, the angular spread is 3.5 degrees (around a meanincidence angle θ=78°). This angle of incidence agrees with thepredicted value for gold-coated silica glass in sucrose solutionsinterrogated at wavelengths close to 1500 nm. The angular spread of theresonance also corresponds well to typical values obtained for SPRmeasurements made with the Kretschmann configuration. Furthermore, thewavelength shift as a function of n_(D) is well approximated by astraight line with a slope of 454 pm/(10⁻³ change in n_(D)). Evenconsidering the dispersion of the sugar solutions between 589 nm and the1520-1560 nm region, this is again in quantitative agreement with theexpected behavior for contra-directional gratings in gold-coated silicafibers where shifts of the order of 100-500 pm/(10⁻³ change in n_(ext))were theoretically predicted. These observations support the hypothesisthat the resonance seen is indeed due to a SPR that is perturbing someof the cladding modes. In particular, the effective indices of theplasmons that are observed are smaller than the glass refractive indexbut larger than the effective indices of the outer medium. Thiscorresponds to a situation where the plasmons are seen as perturbedcladding modes with a local electromagnetic field maximum at the outermetal boundary. It is this local field maximum that enhances thesensitivity of the cladding mode resonance to the exact value ofexternal index.

The SPR waves can be used for chemical and biological monitoring throughchanges in the refractive index of the medium in which the fiber islocated or through changes in the refractive index of the gold layeritself.

The tilted grating discussed above can be used for a number of differentsensing applications, examples of which are discussed below:

1. Strain Gage with Thermal Compensation

-   -   As discussed earlier, the response of the tilted grating        includes a core mode resonance component and a cladding mode        resonance component. The core mode resonance component conveys        the response of the sensor to temperature and elongation strain.        In a specific example as the temperature of the grating changes        or as elongation strain acts on the tilted grating the peak        wavelength of the core mode resonance will shift. On the other        hand, the wavelength gap between the peak wavelength of the core        mode resonance and anyone of the peaks in the cladding mode        resonance is generally constant with temperature. This means        that as the temperature changes this gap will also change.        However, if only elongation strain is applied on the sensor and        the temperature is maintained constant the peaks of the core        mode resonance component and of the cladding mode resonance        components will shift in unison maintaining the gap constant.        So, the gap change is indicative of the temperature variation        only. Once the wavelength shift due only to temperature is        determined, it suffices to subtract this wavelength shift from        say the total wavelength shift of the main peak of the core mode        resonance to determine the wavelength shift due only to        elongation strain. Once this wavelength shift is known, the        elongation strain can be derived easily.    -   FIG. 14 shows a measurement apparatus 1000 using a titled        grating. The measurement apparatus 1000 can be used to measure        elongation strain, independent of temperature variations. The        measurement is performed in a sensing zone 1002. Generally, the        measurement apparatus 1000 has an optical sensor 1004 which is        located in the sensing zone 1002 and includes a tilted grating,        a signal processing device 1006 which performs an analysis of        the optical response generated by the optical sensor 1004, and        an optical excitation generator 1008 that injects into the        optical sensor 1004 an optical excitation.    -   The sensing zone 1002 is the area where the measurement is to be        made. The optical sensor 1004 has a continuous length of optical        fiber. The optical fiber has a core in which is formed a TFBG.    -   In use, the optical excitation generator 1008 generates light        which is injected into the optical fiber length that leads to        the optical sensor 1004. The optical excitation reaches the TFBG        which filters out from the optical excitation wavelengths        corresponding to the peaks in the core and cladding mode        resonances. The optical excitation that reaches the signal        processing device 1006 is lacking the wavelengths filtered out        by the TFBG. The signal processing device 1006 uses the        information it receives from the optical sensor 1004 to derive        the intensity of the elongation strain acting on the optical        sensor 1004 in the sensing zone 1002, corrected for temperature        variations in the sensing zone 1002. If desired to measure        pressure or displacement acting on the optical sensor 1004,        there may be a necessity to mount the optical sensor 1004 on a        transducer structure (not shown in the drawings) that is        directly exposed to pressure or displacement and communicates        this pressure or displacement directly to the optical sensor        1004 in the form of elongation strain. Such transducer        structures are known in the art and do not need to be discussed        here in greater details.    -   Note that different measurement apparatus architectures can be        used. A variant is shown in FIG. 18, where the excitation and        the collection of the TFBG response are made from the same side        of the optical fiber. Specifically, in this embodiment, light        generated by the optical generator 1008 is injected into the        optical fiber length that leads to an optical coupler and to the        optical sensor 1004. The optical excitation reaches the TFBG        which filters out from the optical excitation wavelengths        corresponding to the peaks in the core and cladding mode        resonances. The optical fiber includes a reflector 1017 that        will reflect back toward the signal processing device/optical        excitation generator combination, the response produced by the        TFBG. The reflector 1017 can be formed at a termination point of        the optical fiber. In a specific example of implementation, the        termination is a straight termination. Different mechanisms can        be used to provide reflectivity at the reflector 1017. In one        example, the reflectivity is achieved via Fresnel reflection. In        a different example, the reflectivity is provided by a        reflective coating deposited on the straight termination. In        another possible example, the reflector 1017 is a non-tilted        grating that has a reflection spectrum wide enough to overlap        two or more of the cladding mode resonances produced by the        tilted grating.    -   Another possibility is to form on the sensor a coating which        expands in response to a certain substance or condition. The        expansion causes elongation strain which can then indirectly        either detect the substance or measure its concentration. The        specification provides below examples of substances that respond        to physical manifestations and that can induce a measurable        strain on the sensor.    -   The signal processing device extracts from the response from the        optical sensor 1004 information on the elongation strain acting        on the optical sensor 1004 along with the temperature in the        sensing zone 1002. FIG. 15 is a block diagram of the signal        processing device 1006. The signal processing device 1006 is        based on a computer platform that enables to perform digital        signal processing on the response received from the optical        sensor 1004 such as to derive the information desired. More        specifically, the signal processing device 1006 includes in        input interface 1010 that is coupled to the optical fiber length        leading directly to the optical sensor 1004. The input interface        1010 will convert the signal into an electric digital signal,        including performing appropriate filtering. The digital signal        is then impressed on the data bus 1012 that establishes a        communication path between a processor 1016 and a memory device        1014. The processor 1016 executes program code that processes        the data in the digital signal to extract information on the        elongation strain and temperature, according to the logic        discussed above.    -   The signal processing device 1006 also has an output interface        1018 that allows communicating the result of the mathematical        processing to an external entity. The external entity can be a        human operator or a piece of equipment that uses the information        generated by the signal processing device 1006 for specific        purposes.    -   Accordingly, the measurement apparatus 1000 can measure the        elongation strain acting on the optical sensor 1004 and the        temperature in the sensing zone 1002.

2. Multi-Purpose Sensor

-   -   As discussed previously, the TBFG can be used to sense two or        more physical manifestations. A sensor can be provided to        measure the index of refraction adjacent the sensing surface of        the sensor, and another physical manifestation such as        elongation strain and/or temperature. The measurement of the        elongation strain and/or temperature was discussed earlier. The        measurement of the index of refraction can be made in two        different ways. One is to track a wavelength shift of the        cladding mode resonances, as discussed in connection with FIGS.        2 a and 2 b. The other is to detect the occurrence of surface        plasmon resonances, by determining which one of the cladding        modes will experience loss, as discussed earlier.

3. Bending and Strain Gage and/or Temperature Sensor

-   -   As discussed earlier, the cladding mode resonances of the TFBG        can be used to detect bending, in particular the low order        cladding modes. Accordingly, by interrogating the TFBG in the        wavelength that corresponds to those low order cladding modes,        the degree of bending of the sensor can be determined. In a        specific example, the power ratio of the first two resonances in        the cladding mode varies with the degree of bending and can be        used to accurately measure this parameter. At the same time the        elongation strain and/or temperature can be measured by the        techniques discussed earlier. The measurement of the degree of        bending of the sensor can be used as an indirect measure of        another physical manifestation which acts on the sensor to        induce bending in it. For example, the sensor can be mounted on        a diaphragm that is exposed to pressure. The degree of pressure        determines the extent to which the diaphragm bows out. A sensor        placed on the diaphragm will be caused to bend accordingly, and        by measuring the degree of bending one can determine the extent        to which the diaphragm bows and consequently the pressure acting        on the diaphragm. This is shown in FIG. 16. The diaphragm 2000        that is made of flexible material is exposed to pressure on its        side 2002. A sensor 2004 is mounted to the diaphragm. The sensor        is placed on the convex side of the diaphragm 2000 but it can        also be placed on the concave side as well.    -   Another possibility is to coat the sensor with a substance that        induces bending in the sensor in response to a certain physical        manifestation. For example, the coating can be placed only on        one side of the sensor. The material of the coating is selected        such that it swells when in contact with the substance to        detect. When the coating swells it induces bending in the sensor        which can be detected as indicated earlier. By properly        selecting the coating the sensor can thus be made responsive to        a wide variety of substances, such as humidity (water), chemical        substances and biological substances.    -   In one example, the substances that would be responsive to water        are, but not limited to, water-swellable materials.    -   The water-swellable material relates, for example, to a        particulate absorbent material comprising a particulate core of        absorbent polymers, coated with a coating agent, comprising or        being an organic coating compound, which has one or more polar        groups.    -   The absorbent polymer refers, for example, to a polymer, which        is water-insoluble, water-swellable or gelling. These polymers        are typically lightly cross-linked polymers, which contain a        multiplicity of acid functional groups such as carboxylic acid        groups. Examples of acid polymers suitable for use herein        include those which are prepared from polymerizable,        acid-containing monomers, or monomers containing functional        groups which can be converted to acid groups after        polymerization. Thus, such monomers include olefinically        unsaturated carboxylic acids and anhydrides, and mixtures        thereof. The acid polymers can also comprise polymers that are        not prepared from olefinically unsaturated monomers.    -   Examples of such polymers include, but are not limited to,        polysaccharide-based polymers such as carboxymethyl starch and        carboxymethyl cellulose, and poly(amino acid) based polymers        such as poly(aspartic acid).    -   Some non-acid monomers can also be included, usually in minor        amounts, in preparing the absorbent polymers herein. Such        non-acid monomers can include, for example, but not limited to,        monomers containing the following types of functional groups:        carboxylate or sulfonate esters, hydroxyl groups, amide-groups,        amino groups, nitrile groups, quaternary ammonium salt groups,        and aryl groups (e.g., phenyl groups, such as those derived from        styrene monomer). Other optional non-acid monomers include        unsaturated hydrocarbons such as ethylene, propylene, 1-butene,        butadiene, and isoprene.    -   Olefinically unsaturated carboxylic acid and anhydride monomers        useful herein include the acrylic acids typified by acrylic acid        itself, methacrylic acid, alpha.-chloroacrylic acid,        a-cyanoacrylic acid, beta.-methylacrylic acid (crotonic acid),        .alpha.-phenylacrylic acid, .beta.-acryloxypropionic acid,        sorbic acid, .alpha.-chlorosorbic acid, angelic acid, cinnamic        acid, p-chlorocinnamic acid, .beta.-stearylacrylic acid,        itaconic acid, citroconic acid, mesaconic acid, glutaconic acid,        aconitic acid, maleic acid, fumaric acid, tricarboxyethylene,        and maleic anhydride.    -   In one specific example, the absorbent polymers contain carboxyl        groups. These polymers include hydrolyzed starch-acrylonitrile        graft copolymers, partially neutralized hydrolyzed        starch-acrylonitrile graft copolymers, starch-acrylic acid graft        copolymers, partially neutralized starch-acrylic acid graft        copolymers, hydrolyzed vinyl acetate-acrylic ester copolymers,        hydrolyzed acrylonitrile or acrylamide copolymers, slightly        network cross linked polymers of any of the foregoing        copolymers, polyacrylic acid, and slightly network cross linked        polymers of polyacrylic acid. These polymers can be used either        solely or in the form of a mixture of two or more different        polymers.    -   Those of skills in the art will be familiar with the process of        coating the water-swellable material onto the sensor.    -   In a further example, the water-swellable material includes, but        is not limited to, clay intimately mixed with a polypropene, a        polybutene or a mixture of polypropene and polybutene, and a        clay binding ion-exchange or coupling agent compound, to provide        a composition having an unexpected capacity for swelling upon        contact with water. The composition should include a clay binder        that is ion-exchanged with clay platelet cations on internal        negative charge sites of the clay platelets, or reacted with        hydroxyl moieties at the clay platelet edges to achieve        unexpected water-swellability.    -   The water-swellable clay utilized can be, but not limited to,        any water-swellable layered material, such as a smectite clay,        which will swell upon contact with water. The clay may be        smectite clay, such as a montmorillonite or a bentonite clay.        This clay has sodium as a predominant exchange cation. However,        the clay utilized to may also contain other cations such as        magnesium and iron.    -   Those skilled in the art will be familiar with the methods for        preparation of the compositions described herein.

4. Chemical/Biological Sensor

-   -   The sensor using a TFBG uses an interface responsive to the        biological or chemical element to be detected to produce a        physical manifestation that can be measured by the sensor. The        interface can be designed to impress on the sensor a physical        force which can be directly measured. An example of such        interface was mentioned earlier and it would typically be in the        form of a coating that causes the sensor to bend or stretch when        it comes in contact with the biological or chemical element to        be detected.    -   The interface can also be such as to cause SRI changes in        response to the presence of the biological or chemical element        to be detected. The chemical or biological sensor can also        function without the need of an interface detects SRI changes        which can be used in applications where a direct measure of the        SRI is required or applications where the SRI change is an        indicator of the occurrence of a chemical or a biological        process or element. As briefly mentioned above, the SRI can be        measured in two different manners. One involves tracking the        wavelength shift by of the cladding mode resonances, as        discussed in connection with FIGS. 2 (a) and 2 (b). The other        uses the detection of SPR.    -   The sensor can be used as a chemical sensor to detect changes in        SRI caused by the presence of a chemical element such that, but        not limited to, the sensor can be used for determining the        concentration of sugar in a medium, such as an aqueous solution,        for determining the concentration of alcohol in a medium, for        measuring the degree of curing of an adhesive, as the adhesive        cures the SRI changes. By measuring the SRI one can track the        degree of curing or detect a threshold at which the adhesive is        considered to be cured. The invention is also used for measuring        the degree of curing of cement in a fashion similar to the        curing of an adhesive. The invention is further used as a        biological detector. Generally, the biological detector includes        an interface.    -   As used herein, the term chemical or biological analyte refers        to a chemical or biological element to be detected.    -   The chemical and biological analytes that are contemplated        include, but are not limited to, bacteria; yeasts; fungi;        viruses; rheumatoid factor; antibodies, including, but not        limited to IgG, IgM, IgA and IgE antibodies; carcinoembryonic        antigen; streptococcus Group A antigen; antigen; viral antigens;        antigens associated with autoimmune disease; allergens; tumor        antigens; streptococcus Group B antigen, HIV I or HIV II        antigen; or host response (antibodies) to these and other        viruses; antigens specific to RSV or host response (antibodies)        to the virus; an antibody; antigen; enzyme; hormone;        polysaccharide; protein; prions; lipid; carbohydrate; drug;        nucleic acid; Salmonella species; Candida species, including,        but not limited to Candida albicans and Candida tropicalis;        Salmonella species; Neisseria meningitides groups A, B, C, Y and        W sub 135, Streptococcus pneumoniae; E. coli K1. E. coli;        Haemophilus influenza type B; an antigen derived from        microorganisms; a hapten; a drug of abuse; a therapeutic drug;        environmental agents; and antigens specific to Hepatitis; an        enzyme; a DNA fragment; an intact gene; a RNA fragment; a small        molecule; a metal; a toxin; a nucleic acid; a cytoplasm        component; pili or flagella component; or any other analytes.    -   The analyte of interest would typically be in a carrier medium        which can be solid, gel-like, liquid or gas. For instance        analyte can be detected in a bodily fluid such as mucous,        saliva, urine, fecal analyte, tissue, marrow, cerebral spinal        fluid, serum, plasma, whole blood, sputum, buffered solutions,        extracted solutions, semen, uro-genital secretion, pericardial,        gastric, peritoneal, pleural, throat swab, subfractions thereof,        or other washes and the like. A gaseous medium may be, but not        limited to, air.    -   An aqueous buffered solution may be employed to dilute the        analyte, such an aqueous buffer solution may be lightly or        heavily buffered depending on the nature of the analyte to be        detected. Various buffers may be employed such as carbonate,        phosphate, borate, Tris, acetate, barbital, Hepes, or the like.        Organic polar solvents, e.g., oxygenated neutral solvents, may        be present in amounts ranging from about 0 to 40 volume percent        such as methanol, ethanol, .alpha.-propanol, acetone,        diethylether, or the like.    -   In one specific example, the interface has a recognition analyte        that can be attached on the sensing surface of the sensor and        will associate with the analyte of interest (FIG. 1). When such        binding occurs the SRI changes. The change is specific in that        the SRI acquires a specific value which indicates that the        binding has taken place. The cladding mode resonances        “interrogate” the electric/dielectric interface and when the SRI        acquires the specific value, the energy in at least one of the        resonance will be transferred into a SPR. The energy transfer        will show as a reduced power in that particular cladding mode        resonance, allowing determining that the SRI is at the specific        value indicative of a binding event. Note that since several        resonances interrogate the electric/dielectric interface, this        technique can at least in theory monitor simultaneously for a        set of different specific SRI values, each associated to a given        resonance in the cladding mode resonance component. As such,        several binding events, corresponding to different SRI values,        can be detected simultaneously.    -   The recognition analyte is thus a component of a specific        binding pair and includes, but is not limited to,        antigen/antibody, enzyme/substrate, oligonucleotide/DNA,        chelator/metal, enzyme/inhibitor, bacteria/receptor,        virus/receptor, hormone/receptor, DNA/RNA, or RNA/RNA,        oligonucleotide/RNA, and binding of these species to any other        species, as well as the interaction of these species with        inorganic species.    -   The recognition analyte that is attached to the sensing surface        can be, but is not limited to, toxins, antibodies, antigens,        hormone receptors, parasites, cells, haptens, metabolizers,        allergens, nucleic acids, nuclear analytes, autoantibodies,        blood proteins, cellular debris, enzymes, tissue proteins,        enzyme substrates, coenzymes, neuron transmitters, viruses,        viral particles, microorganisms, proteins, polysaccharides,        chelators, drugs, and any other member of a specific binding        pair. The recognition analyte is specifically designed to        associate with the analyte of interest.    -   The recognition analyte may be passively adhered to the sensing        surface. If required, the recognition analyte may be covalently        attached to the sensing surface of the sensor. The chemistry for        attachment of recognition analyte is well known to those skilled        in the art.    -   Recognition analyte for detection of bacteria may have binding        activity to specifically bind a surface membrane component,        protein or lipid, a polysaccharide, a nucleic acid, or an        enzyme. The analyte, which is specific to the bacteria, may be a        polysaccharide, an enzyme, a nucleic acid, a membrane component,        or an antibody produced by the host in response to the bacteria.        The presence of the analyte may indicate an infectious disease        (bacterial or viral), cancer or other metabolic disorder or        condition. The presence of the analyte may be an indication of        food poisoning or other toxic exposure. The presence of the        analyte may also be an indication of bacterial contamination.    -   A wide range of techniques can be used to apply the recognition        analyte of the interface to the sensing surface of the sensor.        The sensing surface may be, for example but not limited to,        coated with recognition analyte by total immersion in a solution        for a predetermined period of time; application of solution in        discrete arrays or patterns; spraying, ink jet, or other        imprinting methods; or by spin coating from an appropriate        solvent system. The technique selected should minimize the        amount of recognition analyte required for coating a large        number of test surfaces and maintain the stability/functionality        of recognition analyte during application.    -   Attachment of the Recognition Analyte by Self-Assembled        Monolayers on Sensing Surface    -   In a further embodiment, the invention includes attachment of        the recognition analyte into the surface or the interior of the        sensing surface, through self-assembled monolayers.    -   Self-assembled monolayers can be prepared using different types        of molecules and different substrates. Commonly used examples        are, but not limited to, alkylsiloxane monolayers, fatty acids        on oxidic materials and alkanethiolate monolayers. This type of        self-assembled monolayers holds great promise for applications        in several different areas. Some examples of suggested and        implemented applications are, but not limited to, molecular        recognition, self-assembly monolayers as model substrates and        biomembrane mimetics in studies of biomolecules at surfaces,        selective binding of enzymes to surfaces.    -   There are many different systems of self-assembling monolayers        based on different organic components and supports, such as, but        not limited to, systems of alkanethiolates, HS(CH.sub.2).sub.n        R, on gold layers. The alkanethiols chemisorb on the gold        surface from a solution in which the gold layer is immersed, and        form adsorbed alkanethiolates with loss of hydrogen. Adsorption        can also occur from the vapor. Self-assembling monolayers formed        on gold from long-chain alkanethiolates of structure        X(CH.sub.2).sub.n Y(CH.sub.2).sub.m S are highly ordered. A wide        variety of organic functional groups (X,Y) can be incorporated        into the surface or interior of the monolayer.    -   In one example, the self-assembling monolayer is formed of a        carboxy-terminated alkane thiol stamped with a patterned        elastomeric stamp onto a gold surface. The alkanethiol is inked        with a solution of alkanethiol in ethanol, dried, and brought        into contact with a surface of gold. The alkanethiol is        transferred to the surface only at those regions where the stamp        contacts the surface, producing a pattern of self-assembling        monolayer which is defined by the pattern of the stamp.        Optionally, areas of unmodified gold surface next to the stamped        areas can be rendered hydrophobic by reaction with a        methyl-terminated alkane thiol. The details of the method are        well known in the art.    -   The present invention, the self-assembling monolayer has the        following general formula: X is reactive with metal or metal        oxide. For example, X may be asymmetrical or symmetrical        disulfide (—R′SSY′, —RSSY), sulfide (—R′SY′, —RSY), diselenide        (—R′Se—SeY′), selenide (—R′SeY′, —RSeY), thiol (—SH), nitrile        (—CN), isonitrile, nitro (—NO.sub.2), selenol (—SeH), trivalent        phosphorous compounds, isothiocyanate, xanthate, thiocarbamate,        phosphine, thioacid or dithioacid, carboxylic acids, hydroxylic        acids, and hydroxamic acids.    -   R and R′ are hydrocarbon chains which may optionally be        interrupted by hetero atoms and which are preferably        non-branched for the sake of optimum dense packing. At room        temperature, R is greater than or equal to seven carbon atoms in        length, in order to overcome natural randomizing of the        self-assembling monolayer. At colder temperatures, R may be        shorter. In one embodiment, R is —(CH.sub.2).sub.n- where n is        between 10 and 12, inclusive. The carbon chain may optionally be        perfluorinated. A person skilled in the art will understand that        the carbon chain may be of any length.    -   Y and Y′ may have any surface property of interest. For example,        Y and Y′ could be any among the great number of groups used for        immobilization in liquid chromatography techniques, such as        hydroxy, carboxyl, amino, aldehyde, hydrazide, carbonyl, epoxy,        or vinyl groups.    -   Self-assembling monolayers of alkyl phosphonic, hydroxamic, and        carboxylic acids may also be useful for the methods of the        present invention. Since alkanethiols do not adsorb to the        surfaces of many metal oxides, carboxylic acids, phosphonic        acids, and hydroxamic acids may be preferred for X for those        metal oxides.    -   R may also be of the form (CH.sub.2).sub.a-Z—(CH.sub.2).sub.b,        where a.gtoreq.0, B.gtoreq.7, and Z is any chemical        functionality of interest, such as sulfones, urea, lactam, and        the like.    -   The stamp may be applied in air, gel, semi-gel, or under a        fluid, such as water to prevent excess diffusion of the        alkanethiol. For large-scale or continuous printing processes,        it is most desirable to print in air, since shorter contact        times are desirable for those processes.    -   In one specific example the sensor can be used in immunoassay        methods for either antigen or antibody detection. The sensors        may be adapted for use in direct, indirect, or competitive        detection schemes, for determination of enzymatic activity, and        for detection of small organic molecules such as, but not        limited to drugs of abuse, therapeutic drugs, environmental        agents), as well as detection of nucleic acids and        microorganisms.    -   For immunoassays, an antibody may serve as the recognition        analyte or it may be the analyte of interest. The recognition        analyte, for example an antibody or an antigen, should form a        stable, dense, reactive layer on the attachment layer of the        test sensor. If an antigen is to be detected and an antibody is        the recognition analyte, the antibody should be specific to the        antigen of interest; and the antibody (recognition analyte)        should bind the antigen (analyte) with sufficient avidity that        the antigen is retained at the surface of the sensing surface.        In some cases, the analyte may not simply bind the recognition        analyte, but may cause a detectable modification of the        recognition analyte to occur. This interaction could cause an        increase in mass at the test surface or a decrease in the amount        of recognition analyte on the test surface. An example of the        latter is the interaction of a degradative enzyme or analyte        with a specific, immobilized substrate. In this case, one would        see a diffraction pattern before interaction with the analyte of        interest, but the diffraction pattern would disappear if the        analyte were present.    -   In another specific example, the sensor applies to detection of        nucleic acid molecules and nucleic acid probes. Nucleic acids        can be attached to sensing surfaces in an hybridization assays.        A sensing surface such as gold is modified with nucleic acids        via for example, but not limited to, linkers, and blocking        moieties, which serve to shield the nucleic acids from the        sensing surface.    -   “Blocking moieties” are molecules which are attached to the        sensing solid support and function to shield the nucleic acids        from the sensing surface. For the purposes of this invention,        the attachment of a sulfur moiety to a sensing surface, such as        gold, is considered covalent.    -   By “nucleic acids” or “oligonucleotides” herein is meant at        least two nucleotides covalently linked together. A nucleic acid        of the present invention will generally contain phosphodiester        bonds, although in some cases, as outlined below, a nucleic acid        analogs are included that may have alternate backbones,        comprising, for example, phosphoramide, phosphorothioate,        phosphorodithioate, O-methylphosphoroamidite linkages and        peptide nucleic acid backbones and linkages.    -   The nucleic acids of the invention may also be characterized as        “probe” nucleic acids and “target” nucleic acids. These terms        are known in the art. Either probe or target nucleic acids may        be attached to the solid support via linker. In a preferred        embodiment, the probe nucleic acids are attached, via linker        moieties, to the solid support, and the target nucleic acids are        added in solution. The nucleic acid and the probe may be        labeled.    -   Probe nucleic acids or probe sequences are preferably single        stranded nucleic acids. The probes of the present invention are        designed to be complementary to the target sequence, such that        hybridization of the target sequence and the probes of the        present invention occur. This complementarity need not be        perfect; there may be any number of base pair mismatches which        will interfere with hybridization between the target sequence        and the single stranded probe nucleic acids of the present        invention. However, if the number of mutations is so great that        no hybridization can occur under even the least stringent of        hybridization conditions, the sequence is not a complementary        target sequence.    -   It will be appreciated by those in the art, the length of the        probe will vary with the length of the target sequence and the        hybridization and wash conditions.    -   In a possible variant, the nucleic acid is attached to the        sensing surface in monolayers. The techniques to attach nucleic        acid molecules to the sensing surface will be well known to        those skilled in the art.    -   “Target nucleic acids” or “sequences” means a nucleic acid        sequence on a single strand of nucleic acid. The target sequence        may be a portion of a gene, a regulatory sequence, genomic DNA,        cDNA, mRNA, or others. It may be any length, with the        understanding that longer sequences are more specific. As is        outlined herein, probes are made to hybridize to target        sequences to determine the presence or absence of the target        sequence in a sample. Target nucleic acids may be prepared or        amplified as is commonly known in the art. When target nucleic        acids are attached to the sensing surface, they will generally        be the same size as outlined for probe nucleic acids, above.    -   In general, blocking moieties have at least a first and a second        end. The first end is used to covalently attach the blocking        moiety to the sensing surface. The second end terminates in a        terminal group, defined below. However, in some embodiments, the        blocking moieties may be branched molecules. Thus, for example,        the first end is used for attachment to the solid support and        all or some of the other ends may terminate in a terminal group,        as defined below.    -   The second end of the blocking moiety terminates in a terminal        group. By “terminal group” or “terminal moiety” herein is meant        a chemical group at the terminus of the blocking moiety. The        terminal groups may be chosen to modulate the interaction        between the nucleic acid and the blocking moieties, or the        surface. Thus, for example, in another embodiment, when the        blocking moieties form a monolayer as is generally described        below, the terminal group may be used to influence the exposed        surface of the monolayer. Thus, for example, the terminal group        may be neutral, charged, or sterically bulky. For example, the        terminal groups may be negatively charged groups, effectively        forming a negatively charged surface such that when the probe or        target nucleic acid is DNA or RNA the nucleic acid is repelled        or prevented from lying down on the surface, to facilitate        hybridization. This may be particularly useful when the nucleic        acid attached to the sensing surface via a linker moiety is        long.    -   In addition to the blocking moieties, the sensing surface of the        invention comprises modified nucleic acids.    -   The nucleic acids of the invention are modified with linker        moieties, to form modified nucleic acids which are attached to        the sensing surface. By “modified nucleic acid” herein is meant        a nucleic acid as defined above covalently attached to a linker        moiety.    -   By “linker moieties” is meant molecules which serve to        immobilize the nucleic acid at a distance from the sensing        surface. Linker moieties have a first and a second end. The        first end is used to covalently attach the linker moiety to the        sensing surface. The second end is used for attachment to the        nucleic acid.    -   The blocking moieties are made using techniques well known in        the art.    -   In a further variant, the present invention is useful in methods        of assaying for the presence or absence of target nucleic acids        in the sample to be analyzed. Thus, the present invention        provides methods of hybridizing probe nucleic acids to target        nucleic acids. The methods comprise adding or contacting target        nucleic acids to a sensing surface of the invention. The sensing        surface comprises blocking moieties, and modified probe nucleic        acids. The contacting is done under conditions where the probe        and target nucleic acids, if suitably complementary, will        hybridize to form a double-stranded hybridization complex.    -   The assay conditions may vary, as will be appreciated by those        in the art, and include high, moderate or low stringency        conditions as is known in the art. The assays may be done at a        variety of temperatures, and using a variety of incubation        times, as will be appreciated by those in the art. In addition,        a variety of other reagents may be included in the hybridization        assay, including buffers, salts, proteins, detergents or the        like. Positive and negative controls are generally run.    -   In a further specific example, the sensor is used to detect        microorganisms in medium.    -   The term “microorganism” as used herein means an organism too        small to be observed with the unaided eye and includes, but is        not limited to bacteria, a cell, cells, viruses, protozoans,        fungi, and ciliates.    -   In another embodiment, microorganisms such as, but not limited        to, bacteria, bacteriophages, viruses, and cellular analyte can        be detected by sampling the nucleic acids, such as, but not        limited to, nucleotides or polynucleotides that they contain or        release. Microorganisms can also be detected by sampling the        protein, carbohydrates and/or lipids that they contain or        release.    -   In a possible variant, the microorganims may be detected by a        recognition analyte such as not limited to an antibody assembled        as a monolayer on the sensing surface or may be detected by        interacting directly on the bare sensing surface.    -   The embodiments of the invention described herein can be used in        several application areas, for example, but not limited to, for        the quantitative or qualitative determination of chemical,        biochemical or biological analytes in screening assays in        pharmacological research, for real-time binding studies or in        the determination of kinetic parameters in affinity screening or        in research, for DNA and RNA analytics and for the determination        of genomic or proteomic differences in the genome, for the        determination of protein-DNA interactions, for the determination        of regulation mechanisms for mRNA expression and protein        (bio)synthesis, for the determination of biological or chemical        markers, such as mRNA, proteins, peptides or low molecular        organic (messenger) compounds, for the determination of        antigens, pathogens or bacteria in pharmacological product        research and development, for therapeutic drug selection, for        the determination of pathogens, harmful compounds or germs, such        as, but not limited to, salmonella, prions, viruses and        bacteria.    -   Although various embodiments have been illustrated, this was for        the purpose of describing, but not limiting, the invention.        Various modifications will become apparent to those skilled in        the art and are within the scope of this invention, which is        defined more particularly by the attached claims.

The invention claimed is:
 1. An apparatus for measuring at least onephysical manifestation occurring in a medium, said apparatus comprising:(a) a sensor for sensing the physical manifestation, including: (i) asensing surface for exposure to the medium; (ii) an optical pathway, theoptical pathway including a core and a cladding; (iii) a tilted gratingresponsive to electromagnetic radiation propagating in the opticalpathway to induce a propagation of cladding mode electromagnetic wavesin the cladding and core mode electromagnetic waves in the core; (iv)the sensing surface defining an interaction interface between the mediumand the cladding and allowing the physical manifestation to interactwith the cladding mode electromagnetic waves; (b) a measuring devicecoupled to the optical path for sensing the core mode electromagneticwaves and the cladding mode electromagnetic waves that have interactedwith the physical manifestation for deriving information on temperaturein the vicinity of the sensing surface and a physical manifestationmeasurement.
 2. The apparatus as defined in claim 1, wherein the tiltedgrating is in the core.
 3. The apparatus as defined in claim 2, whereinsaid grating induces SPR in proximity to said sensing surface.
 4. Theapparatus as defined in claim 3, wherein said sensing surface includesmetallic material.
 5. The apparatus as defined in claim 4, wherein saidmetallic material is in the form of a coating over said optical pathway.6. The apparatus as defined in claim 5, wherein said optical pathway andsaid metallic coating form a dielectric/electrical conductor interface.7. The apparatus as defined in claim 6, wherein said metallic coating ishomogenous.
 8. The apparatus as defined in claim 6, wherein saidmetallic coating is heterogeneous.
 9. The apparatus as defined in claim6, wherein said optical path includes an optical fiber.
 10. Theapparatus as defined in claim 6, wherein said metallic coating includesgold material.
 11. The apparatus as defined in claim 1, wherein saidphysical manifestation is an SRI change.
 12. The apparatus as defined inclaim 11, wherein said sensor is a biosensor.
 13. The apparatus asdefined in claim 12, wherein said biosensor is responsive to bacteria.14. The apparatus as defined in claim 12, wherein said biosensor isresponsive to a virus.
 15. The apparatus as defined in claim 11, whereinsaid sensor is responsive to one or more chemical compounds.
 16. Theapparatus as defined in claim 11, wherein said sensor is an alcoholsensor.
 17. The apparatus as defined in claim 11, wherein said sensor isa sugar sensor.
 18. The apparatus as defined in claim 11, wherein saidsensor measures a rate of curing of a material.
 19. The apparatus asdefined in claim 18, wherein the material is adhesive.
 20. The apparatusas defined in claim 18, wherein the material is cement.
 21. Theapparatus as defined in claim 11, wherein said sensor measures SRI inthe range from about 1.29 to about 1.45.
 22. The apparatus as defined inclaim 2, wherein the electromagnetic radiation propagates in saidoptical pathway along a propagation direction, said sensor including areflector in said optical pathway to reflect the response of said tiltedgrating.
 23. The apparatus as defined in claim 22, wherein saidreflector directs the response to travel in said optical pathway in adirection opposite to the direction of propagation.
 24. The apparatus asdefined in claim 23, wherein said reflector includes a grating otherthan said tilted grating.
 25. The apparatus as defined in claim 24,wherein said grating other than said tilted grating is a non-tiltedgrating.
 26. The apparatus as defined in claim 25, wherein the responseof said sensor includes a plurality of cladding mode resonances, saidgrating other than said tilted grating has a reflection spectrum thatoverlaps two or more of said cladding mode resonances.
 27. A method fordetecting the presence of bacteria in a medium, comprising: a) providinga sensor having a sensing surface, said sensor having an optical pathwaycontaining a tilted grating, the optical pathway including a core and acladding, the tilted grating being responsive to electromagneticradiation traveling in the optical pathway to induce electromagneticwaves in the cladding susceptible to interact with the medium via thesensing surface; b) placing the sensing surface in contact with themedium; c) detecting a response produced by the sensor which conveysinformation about an interaction between the electromagnetic waves inthe cladding and the medium; and d) determining from the response ifbacteria are present in the medium.
 28. The method as defined in claim27, wherein the tilted grating is in the core.
 29. A sensor for sensingat least one physical manifestation occurring in a medium, said sensorcomprising: a. a sensing surface for exposure to the medium; b. anoptical pathway; c. a tilted grating in said optical pathway, saidgrating being responsive to electromagnetic radiation propagating insaid optical pathway to generate a response conveying information on theat least one physical manifestation; d. the electromagnetic radiationpropagating in said optical pathway along a propagation direction, saidsensor including a reflector in said optical pathway to reflect theresponse of said tilted grating; e. the reflector directing the responseto travel in said optical pathway in a direction opposite to thedirection of propagation; and f. the optical pathway including astraight termination, the reflector being at the termination.
 30. Thesensor as defined in claim 29, wherein said reflector providesreflectivity through Fresnel reflection.
 31. The sensor as defined inclaim 29, wherein said reflector includes a reflective coating depositedon said straight termination.